• carissa184

Roof Decking

Updated: Sep 20, 2021


wood roof decking

While reflective single-ply roof membranes and high-thermal roof insulations seem to have captured current interest in low slope roofing, they are just two of many items to be considered in a roofing system.

Not only that, but roughly two-thirds of all low-slope roofing activity involves replacement or renovation. In these cases, there may be no designer of record, and critical information about the structure and roof may have been lost over time.

Since the foundation of any roof system is the roof deck, this column will focus on several of the roof deck systems that a building manager might encounter when contemplating roof work.

In addition to resisting gravity loads and lateral loading from wind and seismic forces, a building’s structural deck must satisfy these other design requirements:

  • Deflection resistance

  • Component anchorage technique

  • Dimensional stability

  • Fire resistance

  • Surface characteristic

While all these attributes may have been addressed by the building designer on the drawing board, the building manager needs to know what is overhead and what its current condition is. Things may have changed over time. Examples would be the conditions of occupancy, high interior humidity, corrosion of fasteners or the deck itself, installation of new equipment on the roof, types of roof membranes in place (especially if the roof has been re-covered since the original occupancy), and much more.

The basic roof decks commonly used with commercial membrane roofing systems are:

  • Steel – light-gauge, cold-rolled sections, welded or screwed to bar joists (see FIG 1)

  • Wood sheathing – sawed lumber, plywood or OSB (oriented strand board)

  • Concrete – poured-in-place or precast

  • Gypsum – precast or poured-in-place

  • Cementitious wood fiber (see FIG 2)

  • Composite decks of lightweight insulating concrete on corrugated steel or form boards (see FIG 3)

  • Thermoset, compacted asphalt fills

We are fortunate to have trade associations that focus on each of these deck types and provide important documents as free downloads from the internet.

Steel Deck Institute (SDI) http://www.sdi.org

Engineered Wood Association (APA) http://www.apawood.org

National Roof Deck Contractors Assoc. http://www.nrdca@nrdca.org

National Roofing Contractors Association http://www.nrca.net

Single-Ply Roofing Institute http://www.spri.org

Why should a property owner or manager be interested in what deck is on the building and its condition? Among the reasons:

  1. Condition of the roof deck – is it corroded, detached, damaged or unsafe? Has the deck been deformed or deflected to the point where it ponds water? (FIG 4)

  2. Is the deck capable of handling a heavier roof system, such as replacing a built-up roof membrane (4-10 psf) with a ballasted single-ply system (10-20 psf)?

  3. If mechanical fasteners are going to be used to attach insulation or a single-ply membrane to the deck, will the deck provide adequate pullout resistance? Some single-ply systems require a stronger steel deck (80 ksi instead of 33) to meet wind design.

  4. If the roof needs to be patched, can sections of the deck be safely removed and replaced if necessary? (FIG 5) At one time in the past, it was common practice to remove deteriorated gypsum decking and to replace it with metal decking and rigid roof insulation. NRDCA strongly warns that this is not recommended and may be unsafe. New form boards, galvanized wire mesh and poured gypsum should be used. It is critical that the galvanized wire mesh be attached to the existing wire at the perimeter of the patch for safety reasons. See NRDCA 500 – Gypsum Roof Deck Replacement Procedures at http://www.nrdca.org/webfiles/NRDCA 500.pdf

  5. If the existing roofing is to be removed down to the deck, and tapered insulation or a sloped fill is planned to solve ponding problems, will the clearance at access doors, windows, and equipment hatches be imperiled?

  6. Has thought been given to adding PV (photovoltaic) panels to the building roof? If so, how will they be attached to the structure?

Steel Roof Decking The SDI Standard RD1.0-2006 for Steel Roof Deck provides most of the information the building owner will need to know. (http://www.sdi.org/ansi/R1SDIANSI.pdf)

Gauge (thickness), rib width, depth, and configuration: Because of the ribbed configuration of the deck, a leveling board will need to be installed over the roof deck. Rib openings would be:

Type of Deck

Rib Width

Narrow Rib Type A

3/4”

Intermediate Rib Type F

1-3/4”

Wide Rib Type B

2-1/2”

Coating on steel:

  • Zinc (galvanized decking meeting ASTM A653, minimum yield strength 33ksi

  • Painted (shop coat of primer) deck meeting ASTM A1008, @33ksi

Information on the existing deck and its condition can be determined by cutting a hole through the roof membrane, all the way down to the deck. (Make sure you have proper repair materials to fix the hole!)

Thickness of a steel roof deck can be determined at a roof hatch where the edge of the deck may be available for measurement with a micrometer. Flexural strength can be assumed to be 33ksi, unless a new single-ply system is contemplated that requires a higher pull-out-strength. In that case, a pullout test device can confirm adequacy. Other things to be observed are the presence or absence of an air barrier/vapor retarder, type, thickness, and condition of the thermal insulation, etc.

Moisture surveys may be able to reveal whether there is wet insulation, and if so, may be able to provide an indication of whether localized patching or roof replacement is necessary.

Attachment of the vapor retarder, if present, could have been by ribbons of cold adhesive or hot asphalt. Roof fasteners at one time were special clips, driven through tin discs, or fasteners with annular rings on the shank and a large stress plate.

Currently, the use of corrosion-resistant screws and stress plates are in vogue. Most likely, a repair or re-cover roof system would use fasteners that penetrate through the first layer of roof insulation. A cover board or second layer of insulation would then be installed using hot asphalt or low-rise polyurethane adhesive. Information on fastener density and type can be found on FM Globe’s RoofNav program (http://www.fmglobal.com/).

Decks Other Than Steel Wood Sheathing – sawed lumber, plywood or OSB (oriented strand board).

At one time, the use of heavy timber tongue-in-groove decking was common. These decks rarely had roof insulation installed on them, since energy was relatively cheap, so typical constructio